Support subscribe result data

[FZambia]

Adds data field to SubscribeSuccessEvent. This allows to put some custom data on server side to be available only to current connection. I.e. some initial subscription data. Specifically, this makes it possible to use data from Subscribe proxy: https://centrifugal.dev/docs/server/proxy#subscribe-proxy.

[SimonVillage]

centrifuge-dart/lib/src/events.dart

Lines 230 to 232 in aff43ea

	String toString() {
	return 'ServerSubscribeEvent{channel: $channel, isResubscribed: $isResubscribed, isRecovered: $isRecovered}';
	}

Should this change to something like

  String toString() {
    return 'SubscribeSuccessEvent{isResubscribed: $isResubscribed, isRecovered: $isRecovered, data: ${utf8.decode(data, allowMalformed: true)}}';
  }

[FZambia]

Should this change to something like

I think no - this is just a string representation so no need to include all fields to it, specifically data can be non utf-8 (any binary supported here) and several kilobytes long for example.

[FZambia]

Actually just noticed that for ConnectEvent we already have data included in string representation. So let's be consistent – added data to string method for SubscribeSuccessEvent too.